System and method for underwater target detection from broadband target returns

Abstract

The probability of detection of an underwater moving target is enhanced by the selective summing of a multiplicity of reverberation suppressed, moving target detections, each appearing in a different and non-overlapping, narrow frequency band of a unitary, broadband transmission. The motion-sensitive target echoes are the result of a multiple-pulse broadband (SONAR) transmission. These broadband received echoes are bandpass filtered to form a set of narrow (e.g., 50 Hz) frequency bands extending over the full bandwidth of the transmission. These narrow band returns are then matched-filtered against a set of moving target replicas. Selected ones of these band-limited, matched-filtered returns are then summed together. The summed result can contribute to an enhanced probability that a unitary, moving target has been detected.

1 . A method for processing broadband echoes received from a target subject to a multiple pulse, broadband transmission which method comprises the steps of band pass filtering said received signals into a multiplicity of sub-bands of said broadband transmission, matched filter processing each of said sub-band signals to provide sub-band matched filtered outputs, combining selected ones of said outputs to provide at least one combined output, and using said combined output for enhanced detection of said target.  
   
   
       2 . The method of  claim 1  including the step of selecting different outputs for combining.  
   
   
       3 . The method of  claim 1  wherein said selecting step is carried out based upon the amplitude of a response in said outputs representative of a moving target.  
   
   
       4 . The method of  claim 3  wherein said selecting step is carried out by responding to each of said sub-band signals and selecting for combining those of said sub-band signals which indicated collectively said moving target at a certain velocity.  
   
   
       5 . The method of  claim 1  wherein said combining step is carried out by selectively summing different ones of said sub-band signals.  
   
   
       6 . The method of  claim 5  wherein said summing step is carried out on different groups of said sub-band signals to provide a plurality of said summed outputs.  
   
   
       7 . The method of  claim 4  wherein said detection step is carried out by visual inspection of the display of said summed sub-band signals.  
   
   
       8 . The method of  claim 1  wherein said broadband signals are provided by a succession of timed underwater impulse events.  
   
   
       9 . The method of  claim 8  wherein said timed impulse events are provided by an airgun array.  
   
   
       10 . The method of  claim 1  wherein said received signals are provided from one or more of a plurality of beam directions with the aid of a directional hydrophone array, and said steps of separation, processing and summing steps are carried out independently for each beam direction to enable the provision of directionality in said detection step.  
   
   
       11 . In a system for sonar signal processing of returns from broadband transmissions by matched filter processing of said returns in a multiplicity of sub-bands, the improvement comprising at least one summer for summing the outputs from said matched filters to provide at least one summed output for use in improved detection of targets from said returns.  
   
   
       12 . The invention of  claim 11  further comprising means for applying selectively said outputs to said summer.  
   
   
       13 . The invention of  claim 11  further comprising means for detecting said target from summed signals provided by said summer.  
   
   
       14 . The invention of  claim 12  further comprising a plurality of said summers to each of which are connected to different groups of said outputs forming sub-bands in lower and higher frequency portions of said broadband transmission.  
   
   
       15 . The invention of  claim 13  further comprising means for selecting said outputs for application to said summer by connecting only those outputs to said summers having a response greater than a certain threshold.  
   
   
       16 . The invention of  claim 15  which comprises means responsive in displaying of said individual outputs for operating said selecting means, and for displaying said summed outputs for use in detecting said targets.  
   
   
       17 . In a system of sonar signal processing of returns which are divided into return events having different characteristics, the improvement comprising at least one combiner for combining said events to provide a combined output which represents a detection of a target exhibiting an enhanced probability of detection over the probability obtained from individual ones of said events.  
   
   
       18 . A method for processing a broad frequency band echo return from an underwater moving target subjected to an incident transmission provided by a sequence of broadband impulses, said method comprising the steps of bandpass filtering said echo return to provide a multiplicity of sub-band responses, matched-filter processing said responses against a set of moving-target replicas for generating outputs representing initial detection of said moving target in said multiplicity of sub-bands, and combining the outputs of a selected set of said sub-bands to enhance the probability of detection of said moving target.  
   
   
       19 . The method of  claim 18  wherein said combining step is carried out by summing said selected outputs.